[Loadstone] Some suggestions and observations
Kerry and Gemma Hoath
kerryandgemma at gotss.net
Tue May 19 07:59:36 BST 2009
First of all many of your questions can be answered by reading the wikipedia
article on gps, or the simplified introduction to gps linked from that page.
If the gps receiver managed to get a fragment of data from the NMEA
sentences emitted by the gps receiver it would have had a timestamp to set
the phone's time to.
As to the data sent by the gps receiver, you can get a good idea by looking
at the reverse engineering documents on the NMEA spec. These should be on
http://gpsd.berlios .de or google for gpsd nmea documentation.
Data includes lattitude, longitude, speed, heading, and various values such
as qdop received from the satelite.
If you pair your gps unit to your computer, (assuming it can talk bluetooth)
you can actually see the NMEA data coming in over a bluetooth serial port
with a program such as teraterm or similar. this data is not easily
comprehensible. The other way you can do this is to start logging on
loadstone, log for a minute or two then stop logging. copy the log to the pc
and take a look in a text editor.
If static navigation is disabled on a receiver and you are stationary your
position will shift around within 10-15 meters of your current location.
This is due to signal reflections, athmospheric drift and instability of pll
components in the receiver.
Once you start to move at a reasonable speed the gps receiver can calculate
where you are by comparing how the readings change and which direction you
are travelling in.
In fact a gps does not know which direction you are facing only which
direction you are travelling in as it only knows relative data by comparing
the readings it has taken and is now receiving.
This is why turning a gps unit 180 degrees without moving it will not show a
change in direction unless the unit has an accelerometer or similar built
into it and most cheap units don't.
As to tide times, i'm sure if you submitted a patch to the loadstone team or
formula there is a chance that this could be integrated.
After all loadstone is open source and code contributions are allways
this said however; there needs to be demand for a feature and people should
be interested in the feature or there is no point spending the time
it depends on both the receiver and location as to whether Waas/ecnos is
available; and the methods to turn it on on various receivers differ.
To make matters worse, many receivers accept a command but don't return
status to say whether it was successful or not.
----- Original Message -----
From: "Kotian, H P" <hpkotian at rbi.org.in>
To: <loadstone at loadstone-gps.com>
Sent: Tuesday, May 19, 2009 2:12 PM
Subject: [Loadstone] Some suggestions and observations
Thanks for responding to my queries.
1 Precisely, I was trying to bring out the inconsistency of information
given by LS. LS should have picked up at least 1 satellite before it could
synchronies the time and get the almanac. However, LS kept reporting 0 of 7
satellites for a very long time that day.
2 I suppose the satellite would be putting out a continuous beacon. Based on
this data the GPS application would be able to work out the point on earth.
What are the different fields that contains in the beacon. Probably almanac
would be one of the field. Just for information and knowledge.
2 I was stationary for more than 15 minutes under open sky. The minimum
requirement for triangulation is 3 birds. Here I am talking of more than 8
birds. The lat / long kept changing when the number kept changing from
between 8 to 12 satellites.
I reside at Mumbai, India. I am unsure if waas/ecnos correction is available
here. I did try putting it on I did not observe any change in reporting of
lat / long info.
3 I don't know of any formulas to compute the tides however, to my
understanding, when the sun or moon is right overhead it is high tide and
when they are at the back of the sphere it is low tide. The half way time
between sunrise and sun set would be the time of high tide. Similarly it
would work out for the moon too.
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